Bratty little siblings and jerk older siblings are probably because during childhood (and maybe continued to adulthood) this is what we feel.
One aspect I want to touch in sibling relationship is some inferiority or superiority complex as one of the effect of one parent "favoring" one over another, among other. I am trying to have a range of interactions, from a relationship that is supportive, in-between, to antagonistic. Lastly there are also featured siblings with unclear or no blood bond.
Now that you mention it, it IS weird how seldom siblings are utilized in fiction...my guess is because:
1. No shipping potential
2. It means not everyone in the story gets a "unique" origin; which takes a little bit of the fun out of making a new character. ^^;
I'm also guilty of ignoring siblings a lot of the time...many of my MC's do have them, but they're usually supporting characters or background characters. The only time I've given a pair of siblings a starring role in recent years was in LADYBIRD:
And even then, Maeve (older) and Nell have been estranged for years, and have to spend time getting reacquainted. ^^; As far as the plot is concerned, they might as well not be related...it's only really relevant in character interactions, where I can show that they share certain tastes and habits, and that they are especially driven to protect each other, willing to rush into deadly situations without hesitation. I guess that's the fun part about writing close siblings; being able to add little details to show they share a special bond. ^^
Well, I think at least in medieval-style fantasy stories, the evil sibling trope is something that comes out of the history of upper-class families in many societies, where brothers in particular were often at each others' throats for inheritance rights. That dynamic gets deliberately injected into foundational books of the genre and then trickles down into books influenced by those classics.
I think dynamics like that are really interesting if the siblings also love each other. Loving each other and being consciously in competition with one another is an interesting relationship to mine. In the novel I'm posting now, my MC had two cousins who are really his siblings in all but name, and they love each other but have relationships that grow increasingly strained because their goals are at odds.
But I'm also in the middle of writing a novel about a royal succession where the main character is an adviser to one of a set of brothers all vying for the throne. They are constantly scheming, plotting, and backstabbing, and it's fun as hell tbh.
Irl, I have two siblings I'm very close to, and I would never frame one for embezzlement in order to secure my own inheritance (only partially because the inheritance I would be backstabbing them over is primarily a storage building full of broken furniture my mom erroneously insists are priceless antiques).
My story consists of 3 brothers! 2 brothers as twins and 1 younger brother.
While having a story where one of the siblings turned out to be evil is a good story plot, I portrayed a more realistic dynamic between them. They fight and bicker all the time, but they also help each other when needed. They all have different personalities and ways of coping with their strict parents.
Maybe I picked up from my experience regarding their siblings' dynamic (since I have 2 siblings myself).
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I think they're underutilized in fiction because your characters are meant to be by themselves. When learning about the Heroic Journey, two of the most common pieces of fiction references is Harry Potter and Star Wars. Luke and Harry both start out their stories as orphaned only children who are forced away from their aunt and uncle who raised them. They are both later "rewarded" with families at the end of their stories. You see the same trope repeated a lot in stories - bad home life which forces them to leave home and then are rewarded with a shiny new family at the end of the series.
But aside from trying to pair your characters off and making one big "family" at the end, there really is no need to add siblings. Siblings in American TV are written as plot devices for male characters and are used to humanize female characters. For every other function that a family member could do, that's where another character in your main cast steps in.
In my opinion, the best way to write good characters is to let them exist. In my main series, my MC discovers she has a twin brother, a Luke and Leia type situation. Aside from them both being sensitive to abandonment, their mutual love of technology, and a shared mysterious family history - they're two characters living in the same world.
Hm, while there are a few Niji Chikara characters with siblings, most of them end up being minor characters who don't really have much impact on the plot. It's just not the type of story that really cares much about sibling dynamics, I guess.
I've written all sorts of sibling dynamics in other stories though. Siblings who care deeply for each other, sibling rivalries, long lost siblings... like any other relationship between characters, it can come in all sorts of flavors, and can be lots of fun to explore. Agree with what's been said above though that sometimes it just doesn't really add anything to the story, so sometimes it's just better to let siblings remain as minor characters and/or let everyone be an only child.
Despite growing up with 2 brothers, I have never felt the need to give my characters siblings. Generally I think it's because I don't really have a use for them in the story, as cruel as that may sound. The one time I had a sibling dynamic of any kind, was when I was working on a story about a royal/imperial family. Which then off course involved a battle for the throne, so not really a wholesome healthy sibling relationship haha.
I do think that the way me and my brothers interact with eachother has an effect on how I write my character interactions though. I think, for example, how Kyara and Zack jokingly tease each other. The dynamic between Kyara and her Scrapper crew (shown in the short story) is also an example of my own sibling dynamic bleeding into the dialogue.
I feel like a lot of the reason a lack of siblings exists in writing is for the same reason a lot of media “kills” off parents or just doesn't even introduce them if the characters do have them. They get in the way of the story unless they play a specific role like you mentioned or the writer just doesn’t have a use for them. Which is strange because both siblings and parental figures are a presence almost everyone has, even if they’re not biological, so I do think this is a pretty interesting recognition.
I'll admit I'm also very guilty of "killing off the parents" mainly because they otherwise would get in the way of me telling an adventure story feature young adults haha. I do try to make the death/absense of these parent revelant to the characters and their personality. But I do fall into that trope 
I did ironically feel the need to add a parental structure though in the form of Nan's orphanage.
My main characters Bobbie and David are brothers. I try to write them as realistically as possible, considering I have 2 siblings myself 
David played a huge role in raising Bobbie though, so their dynamic is a mix of siblings and parental. It's interesting to write for sure.
For anyone who wants to check out my comic about these brothers 
I grew up with a lot of siblings. I think that is why a lot of my characters have at least one sibling. As well as dynamics like, sister-in-law and adopted sibling. Even in the case with Ken, an only child, there is sort of a brother dynamic with his foster cousin.
I guess the weird thing is that I have little desire to make a character with a lot of siblings, similar to my own family. Tho in an earlier version of Frostpeak West, Ace had like 4 or 5 half-brothers but I ended up changing his backstory. I guess it's easier to keep track of 1 sister. In earlier concepts, Ken was Ace's younger brother.
I honestly have very few stories where there isn't at least one relevant character with a sibling that has at least a little bit of bearing on the plot. I guess it's just a difference between writers, as everyone's experiences affect how they write and imagine worlds. My siblings are my best friends, so I guess it comes through with how I write family dynamics. Even if parents are conveniently absent for the sake of the plot, there is usually at least one sibling. My record is currently six siblings (five brothers and a sister)
Siblings can have unique back stories to each other though. Even when they are raised together, they'll have different experiences in school, struggles, xyz.
I realize that; that's why I put "unique" in quotations. ^^;
The difference is that, 9 times out of 10, a sibling will have some awareness of what's going on in their fellow siblings' lives. They may not know that they were suicidal in high school, for instance, but they'll be able to see their lonely daily activities and their negative attitude while at home.
If anything, if they DON'T notice, it would likely become a point of contention in the story, like a "you call yourself my brother, but you were never there when I needed you; you didn't even care!" sort of thing.
The point is, besides not getting to do things like inventing a brand-new country or family history or fantasy race or social dynamic along with the introduction of such a character, there won't be as many "new" revelations involved in divulging their backstory: there's always going to be that large probability that their sibling is already familiar with what you're about to reveal, and a necessity to write with that familiarity in mind.
Ahh, sorry, guess I just misunderstood the quotations. I would agree there's a good chance of siblings having some inclination what you're going through in a lot of instances, though that likelihood decreases the older the siblings are. (As in my story, "Damsel in the Red Dress" where they are both adults, and her little brother, though they are close emotionally, lives in Mexico, so all sorts of things go down in her neck of the woods in the first book without him even knowing.)
I personally feel like siblings is a dynamic so specific that no other person could possibly fill it (unless they were raised with you, making them your sibling imao):
"There’s nothing there but a Christmas card from Andrew and Jinho, ostentatiously addressed to “Her Majesty, The Queen of Baltimore.”"
*
“Jinho told me to say hello to the Queen for him. Gosh! It’ll be so awesome to see you again. You have no idea how much I’ve missed you since the move. He always says I talk about you like you’re some long-deceased relative and not just a phone call away. But calls are just different. We’ll probably be able to get lunch somewhere, the four of us, at least once before we leave. You, me, Jinho, and Kat. I just realized we haven't even introduced them, yet -"
*
“I’ll text you when we’re back in the country so we can arrange to get lunch - but hopefully sooner if we can. Our schedule has been hectic. P.S. I saw the news report for your exhibit online. I’m totally telling everyone that my older sister is a celebrity! (Jinho says we have to fix your house so we can start name-dropping XD.)”
*
It's just different fam
